1. Field of the Invention
The present invention relates to a method of driving a ferroelectric gate transistor memory cell and, more particularly, to a nondestructive memory information reading method.
2. Description of the Related Art
Conventionally, ferroelectric materials are known to have hysteresis characteristics which can be used in data recording. FIG. 1 is a graph showing the hysteresis characteristics of a ferroelectric, in which an electric field (or voltage) E is plotted along the abscissa, and a poling amount P is plotted along the ordinate. Referring to FIG. 1, by causing digital signals of "1" and "0" to correspond to two poled states A and C when the voltage is 0 V, respectively, this ferroelectric can be used as a memory.
For example, a signal of "1" is stored in such a ferroelectric to set the poled state A. When a read pulse E.sub.r in the positive direction is applied, the poled state changes from A to B and then returns to C. The amount of charges generated at this time from the ferroelectric to flow into a read circuit (not shown) is Q.sub.CB -Q.sub.AB =Q.sub.SW. If the ferroelectric is in the state C with a signal of "0", Q.sub.AB =Q.sub.BA is established and the total charge amount becomes "0". In this case, the ferroelectric can be used as a memory.
An attempt is described in detail in "Science", J. F. Scott and C. A. P. Araujo, VOL. 246, p. 1400 (1989), in which a very thin ferroelectric film (e.g., about 100 to 400 nm) is formed on a Si wafer to form a memory cell, and a memory unit is constituted by a combination of this memory cell and a transistor for selecting this memory cell.
On the other hand, an attempt has been made in which a ferroelectric film is formed on a semiconductor substrate to control a current or resistance. Such conventional attempts are described in detail Kyoyudentai Hakumaku Shyusekika Gijutsu (Ferroelectric Thin-film Integration Technique) chapter 5, section 3, p. 261 to 274, Science Forum Publishing Co.. In these attempts, a semiconductor is formed on a bulk ferroelectric, or a ferroelectric is formed on an n-type semiconductor as shown in FIG. 2.
In the structure shown in FIG. 2, in accordance with the polarization amount P of the ferroelectric, a charge having a polarity opposite to the polarization amount P appears at the interface of the semiconductor. The shift amount of the threshold voltage V.sub.th by the charge by poling is represented as follows. ##EQU1## where C.sub.SiO.sbsb.2 represents the capacitance by a gate oxide film, and Q.sub.P represents the amount of charge by poling.
This structure has been known long since. Recently, however, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 4-192173, a memory constituted by one ferroelectric gate transistor is proposed.
However, in the conventional ferroelectric gate transistor (FGT) having the typical structure in FIG. 2, a voltage must be applied between source and drain to read the charge amount. Since the poling of the ferroelectric is deteriorated by this voltage, nondestructive reading is difficult.